Brass and aluminum cap also includes a mild steel weld-in bung. This is also threaded to be a perfect fit on your Moon oil tank, no modification needed. The thread pitch on these caps is 1-5/16" x 12 tpi. These caps have a vent muffler installed on the bottom to prevent excess gas from spitting out, which can be replaced with a 10-32 set screw if a vent is not desired. These are perfect for either a custom oil or gas tank. The venting is hidden under the brass, leaving a very slick look. Check out the tiny allen hardware, word on the street is that LCFabrications employs tiny children with small hands to assemble these. Made in the USA!